Modern conventional electrical resistivity measurements are grouped into two classes, those injecting electrical currents into the formation by means of electrodes (galvanic logging devices, including lateral, spherically focused, and normal devices) and those using coils (induction logging devices) for creating eddy currents in the formation. The galvanic logging methods are really just developments of the original electrode instrument methods invented by the Schlumberger brothers in the 1920's. (L. Allaud and M. Martin, "Schlumberger, the History of a Technique," Wiley, New York, 1977). The induction logging methods and devices were created by Henri-Georges Doll in the 1940's. (H. G. Doll, Pet. Trans. AIME, 186, 148, 1947).
For the induction logging device the signal measured from a particular formation zone is inversely related to the resistivities in the formation around the borehole and to the resistivities within the borehole. For the galvanic logging devices the signal measured is non-linearly related to the resistivities and the resistivity contrasts of the borehole and the formations surrounding the borehole. At the borehole/formation/invaded zone boundaries, electrical charges arise for galvanic logging devices, while for induction logging devices, the effect is due to the induced current in the borehole/formation/invaded zone media. In the field of electromagnetic surveying, the different situations are usually described as different modes, the transverse electric (TE) mode in the case of the induction devices, and the galvanic or transverse magnetic (TM) mode for the galvanic (electrode) devices.
In the field of surface electromagnetic surveying of the earth, the joint measurement and interpretation of inductive and galvanic (TE and TM) techniques has been investigated in recent years. (T. Eadie, "Detection of Hydrocarbon Accumulations by Surface Electrical Methods--A Feasibility Study," Master's Thesis, University of Toronto, 1980). Based on Eadie's work and the initial application of a numerical interpretation technique referred to as "joint inversion" (K. Vozoff and D. L. B. Jupp, "Joint Inversion of Geophysical Data," Geophys. J.R. astr. Soc., v. 42, pp. 977-991, 1975; D. L. B. Jupp and K. Vozoff, "Resolving Anisotrophy in Layered media by Joint Inversion," Geophys. Pros.v. 25, pp. 460-470, 1977; and A. P. Raiche, D. L. B. Jupp, H. Rutters and K. Vozoff, "The Joint Use of Coincident Loop Transient Electromagnetic and Schlumberger Sounding to Resolve Layered Structures," Geophysics, v. 50, n. 10, pp. 1618-1627, 1985) led to the application of the combined modes (inductive and galvanic) for surface geophysical prospecting (K. -M. Strack, T. Hanstein, K. LeBrocq, D. C. Moss, K. Vozoff and P. A. Wolfgram, "Case Histories of LOTEM Surveys in Hydrocarbon Prospective Areas," First Break, v. 7, n. 12, pp. 467-476, 1989; K. -M. Strack, "Exploration With Deep Transient Electromagnetics," Elsevier, 1992, 373 pp.). However, for borehole applications the combination of the two modes (inductive and galvanic) to characterize the same volume of formation has not been developed. Recently, a paper has indicated that the inversion of induction tool responses alone has become available as a wellsite product as an improved interpretation method. (A. Q. Howard, "A New Invasion Model for Resistivity Log Interpretation," The Log Analyst, pp. 97-110, March-April 1992). Historically, induction and galvanic logging measurements investigating the same part of the formation are usually used in different borehole environments (mud conductivity) (Ellis, "Well Logging for Earth Scientists," Elsevier, (1987)) and for different relative formation resistivities (D. E. Johnson and K. E. Pile, "Well Logging For the Nontechnical Person," pp. 68-70, PennWell, 1988). Based on these considerations, either the induction logging or galvanic logging device is chosen for the borehole/formation resistivity conditions. Rarely are both logging devices (induction and galvanic) which investigate the same or similar volumes of formation run together in the borehole because of the excessive tool string length required and the increased cost. Although sometimes induction and galvanic devices are run in the same borehole, usually in separate runs, their measurements are not generally used to produce a combined interpreted response because either one of the logs produced by the two devices is deemed to exhibit the most applicable response characteristics to each particular section of the borehole and the formation.